Large ac magnetic induction technique



1970 lw. M. Kr-:NDRICK 3,535,597

LARGE AC MAGNETIC INDUCTION TECHNIQUE Filed June 20, 1968 I:.frl g.'

P- P @li/ui@ ATTORNEYS.

United States Patent O1 3,535,597 Patented Oct. 20, 1970 ice 3,535,597 LARGE AC MAGNETIC INDUCTION TECHNIQUE Webster M. Kendrick, 3814 Callaway Ave., Baltimore, Md. 1215 Filed June 20, 1968, Ser. No. 738,633 Int. Cl. H01f 27/08, 27/28 U.S. Cl. S17- 155.5 5 Claims ABSTRACT F THE DISCLOSURE Apparatus for the production of magnetic inductions of kilogauss or higher at frequencies beyond 100 kilohertz over volumes of more than one cubic meter for studies. of plasma confinement and for other situations requiring substantial forces. Such inductions are attained by use of coils made up of a large number of paralleled open loops insulated from one another and individually driven by synchronized generators. Series or parallel tuning is used to minimize the effect of self inductance in the loops while equalization of loop currents is used to minimize the effect of mutual inductance among the loops.

The invention described herein may be manufactured, used and licensed by or for the Government for governmental purposes without the payment to me of any royalty thereon.

The invention relates to coils formed by open loops placed adjacent to and parallel to each other wherein said loops are individually driven by synchronized generators for production of high-intensity magnetic inductions of 10 kilogauss or higher at frequencies beyond 100 kilohertz over volumes of more than one cubic meter for studies of plasma confinement and for other situations requiring substantial forces. Large AC magnetic inductions are useful in plasma research and in other situations where forces need to be exerted on bodies having permanent or induced magnetic moments. Pulsed inductions of megagauss magnitude have been produced over volumes of several cubic centimeter size, while DC inductions at the multi-kilogauss level can be generated over volumes of cubic meter size.

It is an object of the invention to provide a device for producing AC magnetic induction of l0 kilogauss or higher at frequencies beyond 100 kilohertz.

Another object of the invention is a device for producing large AC magnetic inductions wherein the effect of the self inductance thereof and skin effect are minimized.

The invention will be more fully understood and its objects and advantages further appreciated by referring now to the detailed description taken in conjunction with the accompanying drawings in which:

FIG. l is a fragmentary View, partly in cross section, of the invention including a schematic diagram of the excitation means;

FIG. 1a is a fragmentary view illustrating parallel tuning of the loops;

FIG. l2 is a perspective view of one of the loops of the coil of the invention coupled to cooling and excitation means;

FIG. 3 is a schematic diagram illustrating a cooling system for the loops comprising the coil of the invention; and

FIG. 4 is a cross sectional view showing an embodiment wherein the loops of the coils are arranged in bank of concentric loops.

The inductance of a circular conductor loop of radius a, formed of a non-ferrous conductor of diameter d (for d/ 2:1502 and dimensioned in cm.) is approximately where a small frequency dependent change in self inductance arising from skin effect has been neglected. Such a loop of radius a=50 cm. formed of copper wire of diameter d=one cm. has an inductance just under three pH. Its impedance is jwL=106 rad/ s. Therefore, if a current of 1000 amperes is required, 3000 volts is needed if RF resistance is neglected. The impedance is further increased by the effect of mutual inductance in the case of multi-loop coils as illustrated in FIG. 1. This increase in impedance due to the effect of self and mutual inductance is too high and a substantial improvement can be obtained by series or parallel tuning the loop as illustrated by inclusion of capacitor 15, as shown in FIGS. 1, la and 2. The aforementioned loop has a total RF resistance of 10-29 so that at series resonance 10 volts will suflice to drive a current of 1000 amperes with a dissipation of l0 kw. In addition, by refrigeration means, power loss in the coils made of such loops and in the current sources can be substantially decreased.

The aforedescribed loop carrying a current at l03 amperes would generate an axial field of 103 ampere turns per meter. The current can be increased in the loop, but is limited by heating problems. An increase in conductor diameter is also favorable, but a more useful result is obtained by increasing the number of loops. A coil of about 1000 such loops all tuned to the same frequency, substantially arranged as shown in FIG. 1, is required to obtain B=1 wb/m.2. The mutual inductance due to coupling between the loops gives rise to multiple resonances and wherein the critical value of coupling is exceeded, as is true in the coil of the invention, the possibility of minimizing the driving voltage will be lost unless this multiplicity of resonances is avoided which is accomplished by equalizing the current in each loop with that in every other loop through adjustment of the amplitude and phase of the output of the generator which drives it. The desired adjustment may be maintained automatically by cornparison of the current in each loop with a reference signal, and using any error signal that may exist to control the generator.

The role of skin effect in limiting passage of high frequency current in conductors is well known and development of the theory is available in literature.

Since the central portion of the circular loop of solid conductor is virtually unused, the area available for carrying AC current is less than that available when the same loop of solid conductor is carrying an equal DC current, and the resistance is increased. The radio frequency resistance is the same as the DC resistance of a conductor having the same outside dimension but whose thickness is that of a layer called the skin depth. Assume the power supplied to the solid conductor loop is 10 kw which appears as heat in the outer layer of the solid conductor loop. Since the skin depth of the solid conductor loop is 0.16 mm., a loop of tubular conductor of one cm diameter with one mm wall thickness will have an RF resistance very close to that of the solid conductor loop. The heat generated in the outer layer of loop of tubular conductor can then be removed by a cooling medium flowing inside the tube comprising the loop, as shown in FIG. 2.

The well-known heating effect of eddy current must be minimized to alloy the desired performance to be obtained. According to theory presented in the literature, this heating is proportional to the square of the frequency, to the square of the induction, and to the square of the conductor diameter. To attain strong inductions at the highest frequencies, the Litzendraht conductor technique is required. In this technique, the loops are fabricated in a composite manner, each loop being made up of a number of strands insulated from each other except at the ends Where they are connected to the current sources. The

strands are made of a wire of suitably small diameter, and the strands composing a given loop are interwoven or transposed in such a way that the total flux linkage of each strand is substantially equal to all others making up the loop. The strands are assembled with a binder into the desired loop configuration.

Referring now with particularity to FIG. 1 which illustrates an embodiment of the apparatus of the invention for production of large AC magnetic inductions comprising a coil consisting of a multiplicity of open loops 11 of metallic tubing separated from each other on an insulating means 12. Each loop 11 is separately excited by means of a separate vacuum tube generator 13. Each generator 13 is coupled to its respective loop 11 by circuit means consisting of a transformer 14 having its primary connected to the -output of the generator and its secondary to ends 16 and 18 of the respective loop. One terminal of the said secondary is connected to end 18 by conductor means 19 and the other terminal of said secondary to end 16 through series tuning capacitor 15. The separate vacuum tube generators 13 are run in synchronism.

FIG. 2 illustrates the means for separately cooling each open loop 11 of coil 10 wherein 20 is a reservoir of cooling medium having an outlet 20a connected by -means of ceramic conductor 21 through circulating pump 22 to end 16 of the open loop whereby the cooling medium is circulated through the open loop. A return ceramic conductor 21a couples the end 18 of the open loop to input 20b of reservoir 20 whereby the cooling medium is returned to the reservoir for recirculation. FIG. 3 is a diagram showing how the individual open loops 11 of coil 10 are separately cooled by utilizing an input manifold 23 which has its input coupled by conductor means through circulating pump 22 to reservoir 20 and its output coupled to end 16 of the individual open loops 11 by ceramic conductors 24. An output manifold 24 has its input coupled to end 18 of individual loops 11 by means of ceramic conductors 25 and its output coupled to the input of reservoir 20 by means of conductor 26.

In FIG. 4 there is shown an other embodiment of the coil 10 wherein the open tubular loops 11 are arranged in a multilayer configuration, for example, layer 27 may consist of a desired number of open loops 11 aligned in parallel along a longitudinal axis in spaced and insulated relationship to each other. Layer 28 consists of the same number of loops 11 spaced and insulated from each other as layer 27, but concentrically disposed in regard to loops 11 of layer 27. Additional layers of loops 11 may also be thus formed as indicated by loop layer 29. Coil 10 thus formed provides for a better coil factor and a higher concentration of the generated high intensity electromagnetic flux. lRadially disposed insulating supports, as indicated by reference numeral 30, are provided with perforations 31 which space the loops of each layer in spaced relationship to each other and in spaced relationship to the loops of Y adjacent layers.

I claim:

1. A device for producing high intensity AC magnetic inductions at high frequencies comprising, a coil consisting of a multiplicity of open loops of non-ferrous material spaced parallel to each other, electrical insulating means spacing said multiplicity of open loops from each other, a multiplicity of high frequency generators for individually driving the open loops, means for tuning each said loops, a coupling means for each said open loop independently coupling each said open loop to one of said high frequency generators to the exclusion of the others, means preventing multiple resonances, and Imeans for cooling said open loops.

2. A device for producing high intensity AC magnetic inductions at high frequencies as claimed in claim 1, wherein each said open loop comprises a tubular conductor and said coupling means comprises a transformer having a secondary winding coupled to the ends of said open loop and a primary winding coupled to 'a respective high frequency generator.

3, A device for producing high intensity AC magnetic inductions at high frequencies as claimed in clai-m 2 wherein means for preventing multiple resonances comprise means for equalizing the current in each open loop with the current in every other open loop.

4. A device for producing high intensity AC magnetic inductions at high frequencies as claimed in claim 3 wherein said means for cooling said open loops comprises, a reservoir of coolant, an output manifold, conductor -lneans connecting a circulating pump `with said reservoir and the input of the output manifold, ceramic conductor means connecting one end of each said open loops with the output of the output manifold, a return manifold, ceramic conductor means coupling another end of each open loops to the input of the return manifold, and conductor means coupling the output of the return manifold to said reservoir whereby the coolant is separately circulated through each said open loops.

5. A device for producing high intensity AC magnetic inductions as claimed in claim 4 wherein said coil comprises concentric layers of non-ferrous tubular open loops insulated from each other, each said layer consisting of a multiplicity of parallel non-ferrous open loops spaced from and insulated from each other, the open loops of each layer being axially aligned with each other, and radially disposed insulating means supporting said open loops.

References Cited UNITED STATES PATENTS 2,698,384 12/1954 Widere 317-156 X 2,697,167 12/1954 Kerst 317-156 X 3,256,464 6/1966 Stauffer 317-1555 X 3,419,904 l2/1968 Weaver et al. 317-123 LEE T. HIX, Primary Examiner U.S. Cl. XJR. 

